Lubricating oil composition

ABSTRACT

A lubricating oil composition including: a lubricating base oil; (A) a dispersant poly(meth)acrylate compound having a weight average molecular weight of 30,000 to 200,000 in an amount of 1 to 10 mass % on the basis of the total mass of the composition; and (B) a non-dispersant poly(meth)acrylate compound having a weight average molecular weight of 15,000 to 100,000 in an amount of no more than 15 mass % on the basis of the total mass of the composition, wherein the ratio MA/MB of the amount MA of the component (A) to the amount MB of the component (B) is 0.05 to 1; and the ratio MwB/MwA of the weight average molecular weight MwB of the component (B) to the weight average molecular weight MwA of the component (A) is 0.05 to 2.

TECHNICAL FIELD

The present invention relates to a lubricating oil composition, and more specifically, relates to a lubricating oil composition suitable as a common lubricating oil of a wet clutch and a hypoid gear in an automobile drivetrain, use of the wet clutch including being allowed to slip, and the hypoid gear being mainly employed in a final reduction gear.

BACKGROUND ART

An automobile usually includes a differential gear, so as to appropriately distribute and transmit torque from a power source such as an engine to each drive wheel while absorbing a difference of rotation numbers between an inner drive wheel and an outer drive wheel during a turn. A simple differential gear (which may be also referred to as an “open differential”) comprises a pair of side gears being connected to left and right drive wheels respectively and being spaced from each other; pinion gears respectively engaging with both of the pair of side gears; a frame member holding the side gears and the pinion gears such that they can rotate; and a ring gear receiving torque from a power source and rotating with the frame member coaxially with the side gears. When the pinion gears do not rotate, both of the side gears rotate at the same speed. By pinion gears' rotating at a rotation number proportionate to the difference of rotation numbers of the left and right drive wheels, the difference of rotation numbers of the left and right drive wheels is absorbed, which offers smooth driving. The rotation number of the ring gear is equal to the average of rotation numbers of the left and right drive wheels (i.e., the average of rotation numbers of the left and right side gears). Usually, torque from a power source such as an engine is transmitted to the ring gear of the differential gear via a hypoid gear. The hypoid gear makes a plurality of teeth engage at the same time, which offers spread load on teeth and thus greater endurance than common bevel gears. The hypoid gear, though, experiences continuous slip at contact faces between teeth, which results in a very severe lubricating condition. Some differential gears include more complicated mechanics such as a limited slip differential (LSD). For any differential gears, though, a lubricating oil thereof is required to have high anti-wear, anti-seizure, and fatigue life properties, especially for lubrication of the hypoid gear. Thus, in general, a separate lubricating oil different from lubricating oils for other mechanical elements (such as engine oils and transmission oils) is used for lubrication of a differential gear.

CITATION LIST Patent Literature

Patent Literature 1: JP 2009-167277 A

SUMMARY OF INVENTION Technical Problem

As explained above, in automobiles, an output from a transmission is transmitted to drive wheels via a hypoid gear and a differential gear. In general, an automobile automatic transmission (including conventional automatic transmissions comprising a planetary gear mechanism but also continuously variable transmissions and dual clutch transmission, etc.) comprises at least one wet clutch which transmits and cuts off torque from a power source. As explained above, in general, different lubricating oils are used for lubrication of an automatic transmission and for lubrication of a hypoid gear and a differential gear. However, it is advantageous if both of them can be lubricated with a single and common lubricating oil, in view of weight reduction, etc. Such a lubricating oil is required to have anti-wear, anti-seizure, and fatigue life properties required for a gear oil, especially for lubrication of a hypoid gear, as well as shudder prevention properties for a wet clutch which the automatic transmission comprises.

An object of the present invention is to provide a lubricating oil composition having anti-wear, anti-seizure, and fatigue life properties required for a gear oil, especially for lubrication of a hypoid gear, as well as shudder prevention properties for a wet clutch.

Solution to Problem

One embodiment of the present invention is a lubricating oil composition comprising: a lubricating base oil; (A) a dispersant poly(meth)acrylate compound having a weight average molecular weight of 30,000 to 200,000 in an amount of 1 to 10 mass % on the basis of the total mass of the composition; and (B) a non-dispersant poly(meth)acrylate compound having a weight average molecular weight of 15,000 to 100,000 in an amount of no more than 15 mass % on the basis of the total mass of the composition, wherein a ratio MA/MB of the amount MA of the component (A) to the amount MB of the component (B) is 0.05 to 1; and a ratio MwB/MwA of the weight average molecular weight MwB of the component (B) to the weight average molecular weight MwA of the component (A) is 0.05 to 2.

In the present description, “(meth)acrylate” means “acrylate and/or methacrylate”.

In the lubricating oil composition, the lubricating base oil preferably consists of: at least one Group II mineral base oil of API base stock categories, at least one Group III mineral base oil of API base stock categories, at least one Group IV synthetic base oil of API base stock categories, or at least one Group V synthetic base oil of API base stock categories, or any combination thereof, wherein the lubricating base oil has a kinematic viscosity at 100° C. of 2.5 to 4.5 mm²/s.

The lubricating oil composition preferably has a kinematic viscosity at 100° C. of 4.5 to 7.5 mm²/s.

Preferably, the lubricating oil composition further comprises: (C) a phosphorus-containing anti-wear agent in an amount of 200 to 1110 mass ppm in terms of phosphorus on the basis of the total mass of the composition.

Preferably, the lubricating oil composition further comprises: (D) an overbased calcium detergent in an amount of 50 to 300 mass ppm in terms of calcium on the basis of the total mass of the composition, the component (D) comprising an overbased calcium salicylate detergent in an amount of 50 to 300 mass ppm in terms of calcium on the basis of the total mass of the composition.

Preferably, the lubricating oil composition further comprises: (E) a sulfur additive in an amount of 800 to 1300 mass ppm in terms of sulfur on the basis of the total mass of the composition.

Preferably, the lubricating oil composition further comprises: (F) a compound comprising a C₁₀₋₂₄ chain aliphatic hydrocarbyl group and at least one functional group in an amount of 0.1 to 5.0 mass % on the basis of the total mass of the composition, the at least one functional group being selected from an amide bond, an imide bond, and an amino group.

Advantageous Effects of Invention

The present invention makes it possible to provide a lubricating oil composition having anti-wear, anti-seizure, and fatigue life properties required for a gear oil, especially for lubrication of a hypoid gear, as well as shudder prevention properties for a wet clutch.

DESCRIPTION OF EMBODIMENTS

The present invention will be described hereinafter. Expression “A to B” concerning numeral values A and B means “no less than A and no more than B” unless otherwise specified. In such expression, if a unit is added only to the numeral value B, the same unit is applied to the numeral value A as well. A word “or” means a logical sum unless otherwise specified.

<Lubricating Base Oil>

A base oil consisting of at least one selected from a mineral base oil and a synthetic base oil can be used as a lubricating base oil without any specific limitation.

Specific examples of the mineral base oil include: paraffinic or naphthenic mineral base oils obtained by refining lubricant oil fractions through at least one of refining processes such as solvent deasphalting, solvent extraction, hydrocracking, hydroisomerizing, solvent dewaxing, catalytic dewaxing, and hydrorefining, the lubricant oil fractions being obtained by vacuum distillation of reduced crude obtained by atmospheric distillation of crude oil; wax isomerized mineral oils; and base oils produced by a process including isomerizing GTL WAX (gas to liquid wax).

A hydrocracked mineral base oil, and/or a wax isomerized isoparaffinic base oil that is obtained by isomerizing raw material containing 50 mass % or more of petroleum wax or GTL wax (such as Fischer-Tropsch synthetic oil) can be preferably used as the mineral base oil.

Examples of the synthetic base oil include poly-α-olefins (such as ethylene-propylene copolymer, polybutene, 1-octene oligomer, and 1-decene oligomer) or hydrogenated products thereof; monoesters (such as butyl stearate, and octyl laurate); diesters (such as ditridecyl glutarate, bis(2-ethylhexyl) adipate, diisodecyl adipate, ditridecyl adipate, and bis(2-ethylhexyl) sebacate); polyesters (such as trimellitate esters); polyol esters (such as trimethylolpropane caprylate, trimethyloipropane pelargonate, pentaerythritol 2-ethylhexanoate, and pentaerythritol pelargonate); aromatic synthetic oils (such as alkylbenzene, alkylnaphthalene, and aromatic esters); and mixtures thereof.

Preferably, % C_(P) of the mineral base oil is no less than 60, more preferably no less than 65, usually no more than 99, and preferably no more than 95. The mineral base oil having % C_(P) of this lower limit or above makes it possible to improve viscosity-temperature characteristics, thermal oxidation stability, and friction properties. The mineral base oil having % C_(P) of this upper limit or below makes it possible to improve solubility of additives.

Preferably, % C_(A) of the mineral base oil is no more than 2, more preferably no more than 1, further preferably no more than 0.8, and especially preferably no more than 0.5, and even may be 0. The mineral base oil having % C_(A) of this upper limit or below makes it possible to improve viscosity-temperature characteristics, thermal oxidation stability, and fuel efficiency.

Preferably, % C_(N) of the mineral base oil is no more than 40, more preferably no more than 35, preferably no less than 1, and more preferably no less than 4. The mineral base oil having % C_(N) of this upper limit or below makes it possible to improve viscosity-temperature characteristics, thermal oxidation stability, and friction properties. The mineral base oil having % C_(N) of this lower limit or above makes it possible to improve solubility of additives.

In the present description, % C_(P), % C_(N) and % C_(A) mean percentage of the paraffinic carbon number to all the carbon atoms, percentage of the naphthenic carbon number to all the carbon atoms, and percentage of the aromatic carbon number to all the carbon atoms, which are obtained by the method conforming to ASTM D 3238-85 (n-d-M ring analysis), respectively. That is, the above described preferred ranges of % C_(P), % C_(N), and % C_(A) are based on values obtained according to the above method. For example, the value of % C_(N) obtained according to the above method may indicate more than 0 even if a mineral base oil does not contain any naphthenes.

In one preferred embodiment, the lubricating base oil consists of: at least one Group II mineral base oil of API base stock categories, at least one Group III mineral base oil of API base stock categories, at least one Group IV synthetic base oil of API base stock categories, or at least one Group V synthetic base oil of API base stock categories, or any combination thereof. A group II base oil is a mineral base oil having a sulfur content of no more than 0.03 mass %, saturates of no less than 90 mass %, and a viscosity index of no less than 80 and less than 120. A group III base oil is a mineral base oil having a sulfur content of no more than 0.03 mass %, saturates of no less than 90 mass %, and a viscosity index of no less than 120. A group IV base oil is a poly-α-olefin base oil. A group V base oil is an ester base oil.

The kinematic viscosity of the lubricating base oil at 100° C. is preferably no more than 6.0 mm²/s, especially preferably no more than 4.5 mm²/s, preferably no less than 2.0 mm²/s, and especially preferably no less than 2.5 mm²/s. The base oil having a kinematic viscosity of this upper limit or below at 100° C. offers improved low-temperature viscosity characteristics of the lubricating oil composition and improved fuel efficiency. The base oil having a kinematic viscosity of this lower limit or above at 100° C. makes it possible to increase oil film thickness in a transition from hydrodynamic lubrication to boundary lubrication (mixed lubrication), and thus to improve fatigue life and anti-seizure properties. In the present description, “kinematic viscosity at 100° C.” means a kinematic viscosity at 100° C. defined in ASTM D-445.

The kinematic viscosity of the lubricating base oil at 40° C. is preferably no more than 40 mm²/s, more preferably no more than 30 mm²/s, further preferably no more than 25 mm²/s, especially preferably no more than 21 mm²/s, preferably no less than 8.0 mm²/s, more preferably no less than 8.5 mm²/s, and especially preferably no less than 9.0 mm²/s. The lubricating base oil having a kinematic viscosity of this upper limit or below at 40° C. offers improved low-temperature viscosity characteristics of the lubricating oil composition, and improved fuel efficiency. The base oil having a kinematic viscosity of this lower limit or above at 40° C. offers enough oil film formation at a lubricating point, which makes it possible to improve lubricity. In the present description, “kinematic viscosity at 40° C.” means a kinematic viscosity at 40° C. defined in ASTM D-445.

The viscosity index of the lubricating base oil is preferably no less than 95, and especially preferably no less than 100. The base oil having a viscosity index of this lower limit or above makes it possible to improve viscosity-temperature characteristics, thermal oxidation stability, but also anti-wear properties of the lubricating oil composition. The viscosity index in the present description means a viscosity index measured conforming to JIS K 2283-1993.

The pour point of the lubricating base oil is preferably no more than −10° C., more preferably no more than −12.5° C., further preferably no more than −15° C., especially preferably no more than −17.5° C., and most preferably no more than −20.0° C. The pour point beyond this upper limit tends to lead to deteriorated low-temperature fluidity of the entire lubricating oil composition. The pour point in the present description means a pour point measured conforming to JIS K 2269-1987.

The sulfur content in the lubricating base oil is, in view of oxidation stability, preferably no more than 1.5 mass %, and more preferably no more than 1.0 mass %.

In one embodiment, a lubricating base oil (hereinafter may be referred to as “lubricating base oil of the present embodiment”) consisting of: at least one Group II mineral base oil of API base stock categories, at least one Group III mineral base oil of API base stock categories, at least one Group IV synthetic base oil of API base stock categories, or at least one Group V synthetic base oil of API base stock categories, or any combination thereof, wherein the lubricating base oil comprises:

(O1) a Group II base oil or a Group III base oil or a Group IV base oil of API base stock categories or any mixed base oil thereof in an amount of 30 to 95 mass % on the basis of the total mass of the entire lubricating base oil, wherein the base oil (O1) has a kinematic viscosity at 100° C. of 1.7 to 2.7 mm²/s, and a viscosity index of 85 or more; and

(O2) a Group II base oil or a Group III base oil or a Group IV base oil of API base stock categories or any mixed base oil thereof in an amount of 5 to 70 mass % on the basis of the total mass of the entire lubricating base oil, wherein the base oil (O2) has a kinematic viscosity at 100° C. of 3.0 to 10.0 mm²/s, and a viscosity index of 110 or more; and

optionally comprises (O3) a Group V base oil of API base stock categories having a kinematic viscosity at 100° C. of 2.5 to 4.5 mm²/s, and a viscosity index of 120 or more in an amount of no more than 15 mass % on the basis of the total mass of the entire lubricating base oil; and

optionally comprises a base oil other than the base oils (O1), (O2) and (O3) in an amount of less than 5 mass % on the basis of the total mass of the entire lubricating base oil,

wherein the lubricating base oil has a kinematic viscosity at 100° C. of 2.5 to 4.5 mm²/s, may be preferably employed as the lubricating base oil. Using the lubricating base oil of the present embodiment as the lubricating base oil makes it possible to increase oil film thickness in a transition from hydrodynamic lubrication to boundary lubrication (mixed lubrication), and thus to further improve anti-wear, anti-seizure, and fatigue life properties.

The kinematic viscosity of the base oil (O1) at 100° C. is 1.7 to 2.7 mm²/s, preferably no more than 2.3 mm²/s, preferably no less than 2.0 mm²/s, and especially preferably no less than 2.2 mm²/s. The base oil (O1) having a kinematic viscosity of this upper limit or below at 100° C. makes it possible to improve fuel efficiency. The base oil (O1) having a kinematic viscosity of this lower limit or above at 100° C. leads to enough oil film formation at a lubricating point, which makes it possible to improve lubricity.

The viscosity index of the base oil (O1) is no less than 85, preferably no less than 90, and more preferably no less than 100. The base oil (O1) having a viscosity index of this lower limit or over makes it possible to improve fuel efficiency. The upper limit thereof is not specifically restricted, and may be, for example, no more than 140.

The pour point of the base oil (O1) is preferably no more than −20° C., more preferably no more than −30° C., and normally no less than −40° C. The pour point of the base oil (O1) of this upper limit or below leads to good low-temperature viscosity properties.

Preferably, % C_(P) of the base oil (O1) is no less than 60, and more preferably no less than 70, and even may be 100. The base oil (O1) having % C_(P) of this lower limit or over leads to good thermal oxidation stability.

Preferably, % C_(N) of the base oil (O1) is no more than 40, and especially preferably no more than 30, and even may be 0. The base oil (O1) having % C_(N) of this upper limit or below makes it possible to improve viscosity-temperature characteristics, thermal oxidation stability, and fuel efficiency.

In one preferred embodiment, the base oil (O1) is a Group II base oil or a Group III base oil of API base stock categories having a kinematic viscosity at 100° C. of 2.2 to 2.7 mm²/s, a viscosity index of 90 to 130, a pour point of −20 to −40° C., % C_(P) of 60 to 100, and % C_(N) of 0 to 40 (hereinafter may be referred to as “base oil (O1a)”). The base oil (O1) is more preferably a Group II base oil or a Group III base oil of API base stock categories having a kinematic viscosity at 100° C. of 2.2 to 2.3 mm²/s, a viscosity index of 100 to 130, a pour point of −30 to −40° C., % C_(P) of 70 to 100, and % C_(N) of 0 to 30 (hereinafter may be referred to as “base oil (O1b)”). Using such a base oil as the base oil (O1) makes it possible to improve fuel efficiency.

The kinematic viscosity of the base oil (O2) at 100° C. is 3.0 to 10.0 mm²/s, preferably no more than 8.0 mm²/s, especially preferably no more than 6.5 mm²/s, and preferably no less than 3.5 mm²/s. The base oil (O2) having a kinematic viscosity of this upper limit or below at 100° C. makes it possible to improve fuel efficiency. The base oil (O2) having a kinematic viscosity of this lower limit or above at 100° C. offers enough oil film formation at a lubricating point, which makes it possible to improve lubricity.

The viscosity index of the base oil (O2) is no less than 110, and preferably no less than 120. The base oil (O2) having a viscosity index of this lower limit or over makes it possible to improve fuel efficiency. The upper limit thereof is not specifically restricted, and may be, for example, no more than 140.

The kinematic viscosity of the base oil (O3) at 100° C. is 2.5 to 4.5 mm²/s, preferably no more than 3.5 mm²/s, and more preferably no more than 3.0 mm²/s. The base oil (O3) having a kinematic viscosity of this upper limit or below at 100° C. makes it possible to improve fuel efficiency. The base oil (O3) having a kinematic viscosity of this lower limit or above at 100° C. offers enough oil film formation at a lubricating point, which makes it possible to improve lubricity.

The viscosity index of the base oil (O3) is no less than 120, and preferably no less than 130. The base oil (O3) having a viscosity index of this lower limit or over makes it possible to improve fuel efficiency. The upper limit thereof is not specifically restricted, and may be, for example, no more than 190.

The content of the base oil (O1) in the lubricating base oil of the present embodiment is 30 to 95 mass %, and preferably 35 to 90 mass %, on the basis of the total mass of the lubricating base oil.

The content of the base oil (O2) in the lubricating base oil of the present embodiment is 5 to 70 mass %, and preferably 10 to 65 mass %, on the basis of the total mass of the lubricating base oil.

The content of the base oil (O3) in the lubricating base oil of the present embodiment is no more than 15 mass %, and preferably no more than 10 mass %, and even may be 0 mass %, on the basis of the total mass of the lubricating base oil. In the present description, that the content of the base oil (O3) is 0 mass % means that the lubricating base oil does not contain the base oil (O3).

The content of the base oil other than the base oils (O1), (O2) and (O3) in the lubricating base oil of the present embodiment is less than 5 mass %, and preferably less than 1 mass %, and even may be 0 mass %, on the basis of the total mass of the lubricating base oil. In the present description, that the content of the base oil other than the base oils (O1), (O2) and (O3) is 0 mass % means that the lubricating base oil consists of the base oils (O1) and (O2), and (optionally) the base oil (O3).

The content of the lubricating base oil in the lubricating oil composition is normally 60 to 95 mass %, and preferably 65 to 90 mass %, on the basis of the total mass of the lubricating oil composition.

<(A), (B): Poly(meth)acrylate Compound>

The lubricating oil composition of the present invention comprises: (A) a dispersant poly(meth)acrylate compound having a weight average molecular weight of 30,000 to 200,000 (hereinafter may be referred to as “component (A)”) in an amount of 1 to 10 mass % on the basis of the total mass of the composition; and (B) a non-dispersant poly(meth)acrylate compound having a weight average molecular weight of 15,000 to 100,000 (hereinafter may be referred to as “component (B)”) in an amount of no more than 15 mass % on the basis of the total mass of the composition, wherein the ratio MA/MB of the amount MA of the component (A) to the amount MB of the component (B) is 0.05 to 1; and the ratio MwB/MwA of the weight average molecular weight MwB of the component (B) to the weight average molecular weight MwA of the component (A) is 0.05 to 2. Combination of the components (A) and (B) makes it possible to increase oil film thickness in a transition from hydrodynamic lubrication to boundary lubrication (mixed lubrication), and thus to further improve fatigue life, anti-wear, and anti-seizure properties, compared to lubricating oils comprising the component (A) or (B) alone.

The weight average molecular weight of the component (A) is 30,000 to 200,000. The component (A) having a weight average molecular weight within this range makes it easy to increase oil film thickness, and thus to improve fatigue life and anti-seizure properties. From the same viewpoint, the weight average molecular weight of the component (A) is preferably no more than 170,000, and, for example, may be no more than 150,000.

The weight average molecular weight of the component (B) is 15,000 to 100,000. The component (B) having a weight average molecular weight of no less than 15,000 makes it easy for the component (B) to stay in an oil film, which makes it possible to increase oil film thickness. The component (B) having a weight average molecular weight of no more than 100,000 on one hand offers good low temperature viscosity characteristics, and on the other hand makes it possible to increase oil film thickness in a transition from hydrodynamic lubrication to boundary lubrication (mixed lubrication), which offers reduced friction coefficient, and thus improved anti-seizure and fatigue life properties. From the same viewpoint, the weight average molecular weight of the component (B) is preferably no more than 90,000, and, for example, may be no more than 80,000.

The amount of the component (A) is 1 to 10 mass % on the basis of the total mass of the composition. The amount of the component (A) of no less than 1 mass % makes it possible to increase oil film thickness in a transition from hydrodynamic lubrication to boundary lubrication (mixed lubrication), and to reduce friction coefficient, and thus to improve fatigue life, anti-wear and anti-seizure properties. The amount of the component (A) of no more than 10 mass % makes it possible to suppress viscosity decrease induced by shearing, and thus to maintain a thick oil film and thus to further improve anti-seizure and fatigue life properties.

The amount of the component (B) is no more than 15 mass % on the basis of the total mass of the composition. The amount of the component (B) of no more than 15 mass % makes it possible to suppress viscosity decrease induced by shearing, and thus to maintain a thick oil film and thus to further improve anti-seizure and fatigue life properties.

The ratio MA/MB of the amount MA of the component (A) to the amount MB of the component (B) is 0.05 to 1. The ratio MA/MB of no less than 0.05 makes it possible to increase oil film thickness in a transition from hydrodynamic lubrication to boundary lubrication (mixed lubrication), and thus to improve fatigue life, anti-wear and anti-seizure properties. The ratio MA/MB of no more than 1 on one hand offers good low temperature viscosity characteristics, and on the other hand makes it possible to increase oil film thickness in a transition from hydrodynamic lubrication to boundary lubrication (mixed lubrication), and thus to reduce friction coefficient and improve fatigue life.

The ratio MwB/MwA of the weight average molecular weight MwB of the component (B) to the weight average molecular weight MwA of the component (A) is 0.05 to 2. The ratio MwB/MwA of no less than 0.05 makes it easy for the component (B) to stay in an oil film, which makes it possible to increase oil film thickness. The ratio MwB/MwA of no more than 2 on one hand offers good low temperature viscosity characteristics, and on the other hand makes it possible to increase oil film thickness in a transition from hydrodynamic lubrication to boundary lubrication (mixed lubrication), and thus to reduce friction coefficient and improve fatigue life and anti-wear properties.

A dispersant (component (A)) or non-dispersant (component (B)) poly(meth)acrylate compound having a weight average molecular weight as described above may be used as a poly(meth)acrylate compound constituting the component (A) or the component (B). A dispersant poly(meth)acrylate compound comprises a nitrogen atom-containing functional group. A non-dispersant poly(meth)acrylate compound does not comprise a nitrogen atom-containing functional group. The nitrogen content in the component (A) is preferably 200 to 400 mass ppm, and in one embodiment, may be 100 to 350 mass ppm, on the basis of the total mass of the component (A) (100 mass %).

For example, a poly(meth)acrylate compound comprising 10 to 90 mol % of the (meth)acrylate structural units represented by the following general formula (1) on the basis of the total monomer units in the polymer (hereinafter may be referred to as “poly(meth)acrylate compound of the present embodiment”) may be preferably employed as a poly(meth)acrylate compound constituting the component (A) or the component (B):

wherein in the formula (1), R¹ is hydrogen or a methyl group, and R² is a linear or branched chain hydrocarbon group having a carbon number of 1 to 18.

The content of the (meth)acrylate structural units represented by the general formula (1) in the polymer is preferably 10 to 90 mol %, more preferably no more than 80 mol %, further preferably no more than 70 mol %, more preferably no less than 20 mol %, further preferably no less than 30 mol %, and especially preferably no less than 40 mol %, in the poly(meth)acrylate compound of the present embodiment. The content of the (meth)acrylate structural units represented by the general formula (1) on the basis of the total monomer units of the polymer beyond 90 mol % may lead to inferior solubility in the base oil, inferior improvement effect on viscosity-temperature characteristics, and inferior low-temperature viscosity characteristics. The content under 10 mol % may lead to inferior improvement effect on viscosity-temperature characteristics.

The poly(meth)acrylate compound of the present embodiment may be a copolymer comprising another (meth)acrylate structural unit in addition to the (meth)acrylate structural unit represented by the general formula (1). Such a copolymer can be obtained by copolymerizing one or more monomer(s) represented by the following general formula (2) (hereinafter referred to as “monomer (M-1)”), and a monomer other than the monomer (M-1):

wherein in the formula (2), R³ represents a hydrogen atom or a methyl group, and R⁴ represents a linear or branched chain hydrocarbon group having a carbon number of 1 to 18.

In one embodiment, R⁴ may be a C₁₋₅ hydrocarbon group, or a C₆₋₁₈ hydrocarbon group, or any combination thereof.

Any monomer may be combined with the monomer (M-1). For example, a monomer represented by the following general formula (3) (hereinafter referred to as “monomer (M-2)”) is preferable. A copolymer of the monomer (M-1) and the monomer (M-2) is a non-dispersant poly(meth)acrylate viscosity index improver.

wherein in the formula (3), R⁵ represents a hydrogen atom or a methyl group, and R⁶ represents a linear or branched chain hydrocarbon group having a carbon number of no less than 19.

R⁶ in the monomer (M-2) represented by the formula (3) is a linear or branched chain hydrocarbon group having a carbon number of no less than 19 as described above, preferably a linear or branched chain hydrocarbon group having a carbon number of no less than 20, further preferably a linear or branched chain hydrocarbon group having a carbon number of no less than 22, and more preferably a branched chain hydrocarbon group having a carbon number of no less than 24. The upper limit of the carbon number of R⁶ is not specifically restricted. R⁶ is preferably a linear or branched chain hydrocarbon group having a carbon number of 50,000 or less, more preferably a linear or branched chain hydrocarbon group having a carbon number of 500 or less, further preferably a linear or branched chain hydrocarbon group having a carbon number of 100 or less, especially preferably a branched chain hydrocarbon group having a carbon number of 50 or less, and most preferably a branched chain hydrocarbon group having a carbon number of 25 or less.

One preferred example of the poly(meth)acrylate compound of the present embodiment is a comb-shaped poly(meth)acrylate. A comb-shaped poly(meth)acrylate here means a copolymer of the monomer (M-1) and the monomer (M-2), wherein the monomer (M-2) is a macromonomer having R⁶ having a number average molecular weight (Mn) of 1,000 to 50,000 (preferably 1,500 to 20,000, and more preferably 2,000 to 10,000) in the formula (3). Examples of such a macromonomer include a macromonomer derived from a hydrogenated product of a polyolefin obtained by copolymerization of butadiene and isoprene.

In the poly(meth)acrylate compound of the present embodiment, the polymer may comprise one kind of (meth)acrylate structural units corresponding to the monomer (M-2) represented by the general formula (3) alone, or may comprise two or more kinds thereof in combination. The content of the structural units corresponding to the monomer (M-2) represented by the general formula (3) on the basis of the total monomer units of the polymer is preferably 0.5 to 70 mol %, more preferably no more than 60 mol %, further preferably no more than 50 mol %, especially preferably no more than 40 mol %, most preferably no more than 30 mol %, preferably no less than 1 mol %, more preferably no less than 3 mol %, further preferably no less than 5 mol %, and especially preferably no less than 10 mol %. The content of the structural units corresponding to the monomer (M-2) represented by the general formula (3) on the basis of the total monomer units of the polymer beyond 70 mol % may lead to inferior improvement effect on viscosity-temperature characteristics, and inferior low-temperature viscosity characteristics. The content under 0.5 mol % may lead to inferior improvement effect on viscosity-temperature characteristics.

One or more selected from a monomer represented by the following general formula (4) (hereinafter referred to as “monomer (M-3)”), and a monomer represented by the following general formula (5) (hereinafter referred to as “monomer (M-4)”) is/are preferable as the other monomer to be combined with the monomer (M-1). A copolymer of the monomer (M-1) and the monomer(s) (M-3) and/or (M-4) is a dispersant poly(meth)acrylate compound. This dispersant poly(meth)acrylate compound may further contain the monomer (M-2) as a constituting monomer.

wherein in the formula (4), R⁷ represents a hydrogen atom or a methyl group, R⁸ represents an alkylene group having a carbon number of 1 to 18, E¹ represents an amine residue or heterocyclic residue having 1 to 2 nitrogen atoms, and 0 to 2 oxygen atoms, and x represents 0 or 1.

Specific examples of an alkylene group having a carbon number of 1 to 18 represented by R⁸ include ethylene group, propylene group, butylene group, pentylene group, hexylene group, heptylene group, octylene group, nonylene group, decylene group, undecylene group, dodecylene group, tridecylene group, tetradecylene group, pentadecylene group, hexadecylene group, heptadecylene group, and octadecylene group (each alkylene group may be either a linear or branched chain).

Specific examples of a residue represented by E¹ include dimethylamino group, diethylamino group, dipropylamino group, dibutylamino group, anilino group, toluidino group, xylidino group, acetylamino group, benzoylamino group, morpholino group, pyrrolyl group, pyrrolino group, pyridyl group, methylpyridyl group, pyrrolidinyl group, piperidinyl group, piperidino group, quinolyl group, pyrrolidonyl group, pyrrolidono group, imidazolino group, and pyrazinyl group.

In the formula (5), R⁹ represents a hydrogen atom or a methyl group, and E² represents an amine residue or heterocyclic residue having 1 to 2 nitrogen atoms, and 0 to 2 oxygen atoms.

Specific examples of a residue represented by E² include dimethylamino group, diethylamino group, dipropylamino group, dibutylamino group, anilino group, toluidino group, xylidino group, acetylamino group, benzoylamino group, morpholino group, pyrrolyl group, pyrrolino group, pyridyl group, methylpyridyl group, pyrrolidinyl group, piperidinyl group, piperidino group, quinolyl group, pyrrolidonyl group, pyrrolidono group, imidazolino group, and pyrazinyl group.

Preferred specific examples of the monomers (M-3) and (M-4) include dimethylaminomethyl methacrylate, diethylaminomethyl methacrylate, dimethylaminoethyl methacrylate, diethylaminoethyl methacrylate, 2-methyl-5-vinylpyridine, morpholinomethyl methacrylate, morpholinoethyl methacrylate, N-vinylpyrrolidone, and mixtures thereof.

Although the copolymerization molar ratio of the copolymer of the monomer (M-1) and the monomers (M-2) to (M-4) is not specifically restricted, monomer (M-1):monomers (M-2) to (M-4) is preferably approximately 20:80 to 90:10, more preferably 30:70 to 80:20, and further preferably 40:60 to 70:30.

The poly(meth)acrylate compound of the present embodiment may be produced by any method without specific limitation. For example, a non-dispersant poly(meth)acrylate compound can be easily obtained by radical solution polymerization of the monomers (M-1) and (M-2) in the presence of a polymerization initiator (such as benzoyl peroxide). For another example, a dispersant poly(meth)acrylate compound can be easily obtained by polymerizing the monomer (M-1), at least one nitrogen-containing monomer selected from the monomers (M-3) and (M-4), and optionally the monomer (M-2) by radical solution polymerization in the presence of a polymerization initiator.

<(C) Phosphorus-containing Anti-wear Agent>

The lubricating oil composition of the present invention preferably comprises: (C) a phosphorus-containing anti-wear agent (hereinafter may be referred to as “component (C)”) in an amount of 200 to 1110 mass ppm in terms of phosphorus on the basis of the total mass of the composition.

Any phosphorus-containing anti-wear agent used for a lubricating oil may be employed as the component (C) without specific limitation. Examples of such a phosphorus-containing anti-wear agent include phosphoric acid, a compound represented by the following general formula (6), a compound represented by the following general formula (7), and metal salts and ammonium salts thereof. At least one selected from them may be employed. In the present description, any compound containing both phosphorus and sulfur shall not fall under the component (C), but shall fall under (E) a sulfur additive described later.

wherein in the general formula (6), R¹⁰ represents a hydrocarbon group having a carbon number of 1 to 30; R¹¹ and R¹² each independently represents a hydrogen atom or a hydrocarbon group having a carbon number of 1 to 30; and R¹⁰, R¹¹ and R¹² may be the same as or different from each other.

wherein in the general formula (7), R¹³ represents a hydrocarbon group having a carbon number of 1 to 30; R¹⁴ and R¹⁵ each independently represents a hydrogen atom or a hydrocarbon group having a carbon number of 1 to 30; and R¹³, R¹⁴ and R¹⁵ may be the same as or different from each other.

Examples of a hydrocarbon group having a carbon number of 1 to 30 in the general formulae (6) and (7) include an alkyl group, a cycloalkyl group, an alkenyl group, an alkyl-substituted cycloalkyl group, an aryl group, an alkyl-substituted aryl group, and an arylalkyl group. The hydrocarbon group is preferably a C₁₋₃₀ alkyl group or a C₆₋₂₄ aryl group, more preferably a C₃₋₁₈ alkyl group, and further preferably a C₄₋₁₂ alkyl group.

Examples of a salt of a phosphorus compound represented by the general formula (6) or (7) include salts where a part or all of the residual acidic hydrogen is/are neutralized by making the phosphorus compound react with: a metal base such as a metal oxide, a metal hydroxide, a metal carbonate, and a metal chloride; ammonia; or a nitrogen-containing compound such as an amine compound having only (a) C₁₋₃₀ hydrocarbon or hydroxy group-containing hydrocarbon group(s) in the molecule.

Specific examples of a metal constituting a metal salt along with a phosphorus compound represented by the general formula (6) or (7) include an alkali metal such as lithium, sodium, potassium, and cesium, an alkali earth metal such as calcium, magnesium and barium, and a heavy metal such as zinc, copper, iron, lead, nickel, silver, and manganese. Among them, an alkali earth metal such as calcium and magnesium, or zinc is preferable. In the present description, “alkali earth metal” shall encompass magnesium.

Preferred examples of a nitrogen compound constituting an ammonium salt together with a phosphorus compound represented by the general formula (6) or (7) include a primary amine, a secondary amine, a tertiary amine having one or two methyl group(s) bonded to the nitrogen atom, and alkanolamine, which are especially preferably an aliphatic amine having a C₁₀₋₂₀ linear or branched chain alkyl or alkenyl group such as decylamine, dodecylamine, dimethyldodecylamine, tridecylamine, heptadecylamine, octadecylamine, oleylamine, and stearylamine.

In one preferred embodiment, among the above, at least one compound of the general formula (6) wherein R¹⁰ and R¹¹ are each independently a C₁₋₃₀ hydrocarbon group and R¹² is hydrogen or a C₁₋₃₀ hydrocarbon group, or phosphoric acid, or any combination thereof may be preferably employed as the component (C). In such an embodiment, the C₁₋₃₀ hydrocarbon group is preferably a C₁₋₃₀ alkyl group or a C₆₋₂₄ aryl group. The aryl group may have at least one alkyl substituent. The carbon number of the alkyl group is more preferably 3 to 18, and further preferably 4 to 12. The carbon number of the aryl group is more preferably 6 to 12, and further preferably 6 to 10.

The content of the component (C) in the lubricating oil composition is preferably 200 to 1110 mass ppm on the basis of the total mass of the composition in terms of phosphorus. The content of the component (C) of this lower limit or over offers improved anti-wear, anti-seizure, and fatigue life properties, and improved shudder prevention lifetime of a wet clutch. The content of the component (C) of this upper limit or under offers further improved anti-seizure, anti-wear, and fatigue life properties, and offers further improved shudder prevention of a wet clutch.

<(D) Overbased Calcium Detergent>

The lubricating oil composition of the present invention preferably comprises an overbased calcium detergent (hereinafter may be referred to as “component (D)”) in an amount of 50 to 300 mass ppm in terms of calcium on the basis of the total mass of the composition.

Any known overbased calcium detergent such as an overbased calcium sulfonate detergent, an overbased calcium phenate detergent, and an overbased calcium salicylate detergent may be preferably employed, and an overbased calcium salicylate detergent may be especially preferably employed as the component (D). The lubricating oil composition containing an overbased calcium salicylate detergent as the component (D) offers further improved fatigue life, anti-wear, and anti-seizure properties, and shudder prevention and shudder prevention lifetime of a wet clutch.

Preferred examples of an overbased calcium sulfonate detergent include: overbased salts of calcium salts of alkylaromatic sulfonic acids obtainable by sulfonating alkylaromatic compounds. The weight average molecular weight of the alkylaromatic compounds is preferably 400 to 1500, and more preferably 700 to 1300.

Examples of the alkylaromatic sulfonic acids include what is called a petroleum sulfonic acid and a synthetic sulfonic acid. Examples of the petroleum sulfonic acid here include a sulfonated product of an alkylaromatic compound of a lubricant oil fraction derived from a mineral oil, and what is called mahogany acid, which is side product of a white oil. Examples of the synthetic sulfonic acid include sulfonated products of alkylbenzene having a linear or branched alkyl group, which is obtainable by: recovering side product in a manufacturing plant of alkylbenzene, which is raw material of detergents; or alkylating benzene with a polyolefin. Another example of the synthetic sulfonic acid is a sulfonated product of alkylnaphthalenes such as dinonylnaphthalene. A sulfonating agent used when sulfonating these alkylaromatics is not specifically limited. For example, fuming sulfuric acid or sulfuric anhydride may be used.

Preferred examples of an overbased calcium phenate detergent include overbased salts of calcium salts of compounds having the structure represented by the following general formula (8):

wherein in the formula (8), R¹⁶ is a C₆₋₂₁ linear or branched chain, saturated or unsaturated alkyl or alkenyl group; m is a polymerization degree, representing an integer of 1 to 10; A is a sulfide (—S—) group or methylene (—CH₂—) group; and y is an integer of 1 to 3. R¹⁶ may be combination of at least two different groups.

The carbon number of R¹⁶ in the formula (8) is preferably 9 to 18, and more preferably 9 to 15. R¹⁶ having less than 6 carbons may lead to inferior solubility in a base oil. R¹⁶ having more than 21 carbons may lead to difficulty in production or inferior thermal stability.

The polymerization degree m in the formula (8) is preferably 1 to 3. The polymerization degree m within this range offers improved thermal stability.

Preferred examples of an overbased calcium salicylate detergent include overbased salts of calcium salicylates. Preferred examples of calcium salicylates include compounds represented by the following general formula (9):

wherein in the formula (9), R¹⁷ each independently represents a C₁₄-C₃₀ alkyl or alkenyl group, and n represents 1 or 2, which is preferably 1. When n=2, R¹⁷ may be combination of different groups.

A method for producing the calcium salicylate is not specifically restricted, and any known method for producing monoalkylsalicylates may be employed. For example, the calcium salicylate can be obtained by: making a calcium base such as an oxide and a hydroxide of calcium react with monoalkylsalicylic acid; or once converting monoalkylsalicylic acid to an alkali metal salt such as a sodium salt and a potassium salt, and then performing a metal exchange reaction with a calcium salt; or the like. The monoalkylsalicylic acid can be obtained by: alkylating a phenol as starting material with an olefin, and then carboxylating the alkylated product with carbonic acid gas or the like; or alkylating salicylic acid as starting material with an equivalent of the olefin; or the like.

A method for obtaining an overbased calcium sulfonate, phenate or salicylate is not specifically limited. For example, a calcium sulfonate, phenate or salicylate may be made to react with a calcium base such as calcium hydroxide in the presence of carbonic acid gas, to obtain an overbased calcium sulfonate, phenate or salicylate.

The base number of the component (D) is preferably 150 to 500 mgKOH/g, and more preferably 200 to 450 mgKOH/g. In the present description, the base number means a base number measured by the perchloric acid method conforming to JIS K2501. Generally, metallic detergents are obtained by reaction in diluents such as solvents and lubricating base oils. Therefore, metallic detergents are on the market as diluted in diluents such as lubricating base oils. In the present description, a base number of a metallic detergent shall mean a base number as containing the diluent.

The metal ratio of the component (D) is preferably 3.0 to 15.0, and more preferably no less than 5.0. The metal ratio of the component (D) is calculated in accordance with the following formula:

metal ratio of component (D)=2×Ca content in component (D) (mol) soap group content in component (D) (mol)

when the component (D) contains two or more different soap groups, “soap group content in component (D) (mol)” is the total molar amount of all the soap groups contained in the component (D).

The content of the component (D) in the lubricating oil composition is preferably 50 to 300 mass ppm on the basis of the total mass of the lubricating oil composition in terms of calcium. The content of the component (D) of this lower limit or over offers further improved fatigue life and anti-seizure properties. The content of the component (D) of this upper limit or below offers further improved anti-wear and anti-seizure properties.

<(E) Sulfur Additive>

The lubricating oil composition of the present invention preferably comprises: (E) a sulfur additive (hereinafter may be referred to as “component (E)”) in an amount of 800 to 1300 mass ppm in terms of sulfur on the basis of the total mass of the composition.

Any known sulfur-containing compound such as sulfurized fat, sulfurized fatty acid, sulfurized ester, sulfurized olefin, dihydrocarbyl (poly)sulfide, thiadiazole compounds, alkylthiocarbamoyl compounds, thiocarbamate compounds, thioterpene compounds, dialkyl thiodipropionate compounds, sulfurized mineral oils, zinc dithiocarbamate compounds, molybdenum dithiocarbamate compounds, molybdenum dithiophosphate compounds, and alkyloxy- or alkenyloxy-substituted cyclic sulfone compounds may be employed as the component (E). One of these sulfur-containing compounds may be used alone, or two or more of them may be used in combination.

Sulfurized fat is a product obtainable by reacting a fat (such as lard oil, whale oil, vegetable oil, and fish oil) with sulfur or a sulfur-containing compound. The content of sulfur in sulfurized fat is not specifically limited, and usually 5 to 30 mass %.

As sulfurized fatty acid, a product obtainable by sulfurizing an unsaturated fatty acid by any method may be employed, and examples thereof include sulfurized oleic acid.

As sulfurized ester, a product obtainable by sulfurizing an unsaturated fatty acid ester (such as a product obtained by reacting any alcohol with an unsaturated fatty acid (such as oleic acid, linoleic acid, or a fatty acid extracted from animal and vegetable fats and oils as described above) by any method may be employed, and examples thereof include: sulfurized methyl oleate, and sulfurized rice bran fatty acid octyl ester.

Examples of sulfurized olefin include compounds represented by the following general formula (10). These compounds can be obtained by reacting a C₂₋₁₅ olefin or a dimer to tetramer thereof with a sulfurizing agent such as sulfur or sulfur chloride. Preferred examples of the olefin include propylene, isobutene, and diisobutene.

R¹⁸-S_(a)-R¹⁹   (10)

wherein in the general formula (10), R¹⁸ represents a C₂₋₁₅ alkenyl group, R¹⁹ represents a C₂₋₁₅ alkyl or alkenyl group, and a represents an integer of 1 to 8.

Dihydrocarbyl (poly)sulfide is a compound represented by the following general formula (11). In the formula (11), when R²⁰ and R²¹ are alkyl groups, the compound may be referred to as an alkyl sulfide.

R²⁰-S_(b)-R²¹   (11)

wherein in the general formula (11), R²⁰ and R²¹ may be the same as or different from each other, and each independently represents a C₁₋₂₀ alkyl group (which may be a linear or branched chain, or may have a ring structure), a C₆₋₂₀ aryl group, a C₇₋₂₀ alkylaryl group, or a C₇₋₂₀ arylalkyl group, and b represents an integer of 1 to 8.

Examples of thiadiazole compounds include 1,3,4-thiaziazole compounds represented by the following general formula (12), 1,2,4-thiaziazole compounds represented by the following general formula (13), and 1,2,3-thiaziazole compounds represented by the following general formula (14):

wherein in the general formulae (12) to (14), R²² and R²³ may be the same as or different from each other, and each independently represents hydrogen or a C₁₋₂₀ hydrocarbyl group; and c and d may be the same as or different from each other, and each independently represents an integer of 0 to 8.

Examples of alkyl thiocarbamoyl compounds. include compounds represented by the following general formula (15):

wherein in the general formula (15), R²⁴ to R²⁷ may be the same as or different from each other, and each independently represents a C₁₋₂₀ alkyl group, and e represents an integer of 1 to 8.

Examples of alkyl thiocarbamate compounds include compounds represented by the following general formula (16):

wherein in the general formula (16), R²⁸ to R³¹ may be the same as or different from each other, and each independently represents a C₁₋₂₀ alkyl group, and R³² represents a C₁₋₁₀ alkylene group.

Examples of thioterpene compounds include a reaction product of phosphorus pentasulfide and pinene.

Examples of dialkyl thiodipropionate compounds include dilauryl thiodipropionate, and distearyl thiodipropionate.

The sulfurized mineral oil is a material obtainable by dissolving elemental sulfur in a mineral oil. The mineral oil used for the sulfurized mineral oil is not specifically limited, and examples thereof include paraffinic mineral oil and naphthenic mineral oil obtained by refining, a lubricant oil fraction obtainable by topping and vacuum distillation of a crude oil, by suitably combined known refining processes. The elemental sulfur may be any of massive, powder, or melt. The content of sulfur in the sulfurized mineral oil is not specifically limited, and is usually 0.05 to 1.0 mass % on the basis of the total mass of the sulfurized mineral oil.

As a zinc dithiocarbamate compound, a compound represented by the following general formula (17) may be employed, and as a molybdenum dithiocarbamate compound, a compound represented by the following general formula (18) may be employed:

wherein in the general formula (17), R³³ to R³⁶ may be the same as or different from each other, and each independently represents a hydrocarbyl group having a carbon number of no less than 1; and

wherein in the general formula (18), R³⁷ to R⁴⁰ may be the same as or different from each other, and each independently represents a hydrocarbyl group having a carbon number of no less than 1, and X¹ to X⁴ each independently represents an oxygen atom or a sulfur atom.

As a molybdenum dithiophosphate compound, a compound represented by the following general formula (19) may be employed. As described above, in the present description, any compound containing both phosphorus and sulfur such as molybdenum dithiophosphate shall not contribute to the amount of the component (C), but shall only contribute to the amount of the component (E).

wherein in the general formula (19), R⁴¹ to R⁴⁴ may be the same as or different from each other, and each independently represents a hydrocarbyl group having a carbon number of no less than 1, and X⁵ to X⁸ each independently represents an oxygen atom or a sulfur atom.

Examples of alkyloxy- or alkenyloxy-substituted cyclic sulfone compounds include compounds represented by the following general formula (20):

wherein in the general formula (20), the R⁴⁵O-substituent may be in any position; R⁴⁵ represents a C₄₋₃₀ alkyl or alkenyl group; and f represents an integer of 1 or 2, which is preferably 1.

R⁴⁵ may be a single group, or any combination of two or more groups. The average carbon number of R⁴⁵ (arithmetic mean of the carbon numbers) is preferably 4 to 22, and more preferably 7 to 13. In one preferred embodiment, the R⁴⁵O-group is substituted in the 3-position.

The content of the component (E) in the lubricating oil composition is preferably 800 to 1300 mass ppm, and more preferably no more than 1200 mass ppm, on the basis of the total mass of the composition in terms of sulfur. The content of the component (E) of this lower limit or over offers further improved anti-wear and anti-seizure properties. The content of the component (E) of this upper limit or below offers further improved oxidation stability and shudder prevention lifetime of a wet clutch.

<(F) Ashless Friction Modifier>

The lubricating oil composition of the present invention preferably comprises: (F) a compound comprising a C₁₀₋₂₄ chain aliphatic hydrocarbyl group and at least one functional group (hereinafter may be referred to as “component (F)”) in an amount of 0.1 to 10.0 mass % on the basis of the total mass of the composition, the at least one functional group being selected from an amide bond, an imide bond, and an amino group. Such a compound functions as an ashless friction modifier. One compound may be used alone, or two or more compounds may be used in combination, as the component (F). A molecule of the component (F) may comprise only one C₁₀₋₂₄ chain aliphatic hydrocarbyl group, or may comprise at least two C₁₀₋₂₄ chain aliphatic hydrocarbyl groups. The molecule may comprise only one functional group or may comprise at least two functional groups, the functional group(s) being selected from (an) amide bond(s), (an) imide bond(s), and (an) amino group(s).

A C₁₀₋₂₄ chain aliphatic hydrocarbyl group in the component (F) is preferably an alkyl or alkenyl group; in one typical embodiment, is a linear chain alkyl or alkenyl group; and in another typical embodiment, a group obtainable by substituting 1 to 5 (preferably 1 to 3) hydrogen atom(s) bonded to (a) carbon atom(s) other than an end carbon (ω-position) of a linear chain alkyl or alkenyl group with (a) methyl group(s). In one embodiment, the carbon number of the hydrocarbyl group is 12 to 18.

Typical examples of the component (F) include compounds represented by the following general formulae (21) to (24), and a compound obtainable by partly acylating nitrogen atoms of a polyamine having 3 to 11 nitrogen atoms with an acylating agent having a C₁₀₋₂₄ chain aliphatic hydrocarbyl group (hereinafter may be referred to as “acylated polyamine”):

wherein in the general formulae (21) and (22), R⁴⁶ is a C₁₀₋₂₄ chain aliphatic hydrocarbyl group; and R⁴⁷ and R⁴⁸ are each independently a hydrogen atom, a C₁₋₅ alkyl or hydroxyalkyl group, or a C₁₀₋₂₄ chain aliphatic hydrocarbyl group.

In the general formulae (21) and (22), a preferred embodiment of “C₁₀₋₂₄ chain aliphatic hydrocarbyl group” is as described above. In one embodiment, a compound of the general formula (22) wherein R⁴⁷ is a C₁₀₋₂₄ chain aliphatic hydrocarbyl group and R⁴⁸ is C₁₋₅ hydroxyalkyl group may be preferably employed among the compounds of the general formulae (21) and (22).

wherein in the general formulae (23) and (24), R⁴⁹ and R⁵⁰ are each independently a C₁₀₋₂₄ chain aliphatic hydrocarbyl group; g and h are each independently an integer of 1 to 5, which is preferably 1 to 3, and in one embodiment 1.

In the general formulae (23) and (24), a preferred embodiment of “C₁₀₋₂₄ chain aliphatic hydrocarbyl group” is as described above. A compound represented by the general formula (23) or (24) can be obtained by reacting a substituted succinic acid anhydride having a C₁₀₋₂₄ chain aliphatic hydrocarbyl group with a polyamine (such as diethylenetriamine). Usually, a mixture of a compound of the formula (23) and a compound of the formula (24) is obtained. The mixture may be employed as the component (F). In the mixture, the molar amount of the compound of the formula (24) is preferably greater than that of the compound of the formula (23). For example, only the compound of the formula (24) may be obtained by a purification method such as column chromatography, and may be employed as the component (F).

Concerning the acylated polyamine, preferred examples of the polyamine having 3 to 11 nitrogen atoms include linear or branched chain polyamines such as diethylenetriamine, triethylenetetramine, tetraethylenepentamine, and pentaethylenehexamine. The branched chain polyamine is a structural isomer of the linear chain polyamine, and has at least one tertiary amino group. One polyamine may be used alone, or two or more polyamines may be used as a mixture. The polyamine preferably has 3 to 6 nitrogen atoms, especially preferably has 4 to 6 nitrogen atoms.

Concerning the acylated polyamine, as the acylating agent having a C₁₀₋₂₄ chain aliphatic hydrocarbyl group, one acylating agent may be used alone, or two or more acylating agents may be used in combination. Preferred examples of the acylating agent having a C₁₀₋₂₄ chain aliphatic hydrocarbyl group include acylating agents represented by the following general formula (25):

wherein in the general formula (25), R⁵¹ is a C₁₀₋₂₄ chain aliphatic hydrocarbyl group; and L is a leaving group which leaves upon a reaction with an amino group of a polyamine.

In the general formula (25), a preferred embodiment of “C₁₀₋₂₄ chain aliphatic hydrocarbyl group” is as described above. Such an acylating agent can be derived from a carboxylic acid R⁵¹-CO₂H by a known method. The acylating agent may be an acid halide (in the formula (25), L is Cl, Br or I), or may be an active ester (such as an ester of a carboxylic acid R⁵¹-CO₂H and N-hydroxysuccinimide). If an acid forms in the system accompanying progress of the acylation reaction, the reaction may be carried out under coexistence of a suitable base in the system. In the acylated polyamine, the ratio of acylated nitrogen atoms among all the nitrogen atoms of the polyamine is preferably 30 to 90%, more preferably 40 to 90%, and further preferably 40 to 85%.

A preferred embodiment of the acylated polyamine is a compound represented by the following general formula (26). In the following general formulae (26) to (28), a preferred embodiment of “C₁₀₋₂₄ chain aliphatic hydrocarbyl group” is as described above.

wherein in the general formula (26), repeating units may be in any order; R⁵² and R⁵³ are each independently a C₁₀₋₂₄ chain aliphatic hydrocarbyl group; Z¹ is a group represented by the following general formula (27); Z² is a group represented by the following general formula (28); i is an integer of 1 to 4, which is preferably 1 to 3; j and k are each independently an integer of 0 to 5, which is preferably 0 to 3; j+k is an integer of 1 to 5, which is preferably 1 to 3; and i+j+2 k is an integer of 1 to 9, which is preferably 1 to 5, and especially preferably 2 to 4.

wherein in the general formula (27), R⁵⁴ is a C₁₀₋₂₄ chain aliphatic hydrocarbyl group.

wherein in the general formula (28), R⁵⁵ is a C₁₀₋₂₄ chain aliphatic hydrocarbyl group; and p is an integer of 1 to 3, which is preferably 2.

The content of the component (F) in the lubricating oil composition is preferably 0.1 to 10.0 mass %, more preferably 0.5 to 5.0 mass %, and especially preferably 0.8 to 4.0 mass %, on the basis of the total mass of the composition.

The content of the component (F) of this lower limit or over offers further improved shudder prevention and shudder prevention lifetime of a wet clutch, and further improved fatigue life properties. The content of the component (F) of this upper limit or under makes it possible to suppress deterioration of low temperature viscosity characteristics and oxidation stability.

<(G) Ashless Dispersant>

The lubricating oil composition of the present invention preferably comprises (G) an ashless dispersant (hereinafter may be referred to as “component (G)”).

For example, at least one compound selected from the following (G-1) to (G-3) may be employed as the component (G):

(G-1) succinimide having at least one C₄₀₋₄₀₀ alkyl or alkenyl group in its molecule, or derivatives thereof (hereinafter may be referred to as “component (G-1)”);

(G-2) benzylamine having at least one C₄₀₋₄₀₀ alkyl or alkenyl group in its molecule, or derivatives thereof (hereinafter may be referred to as “component (G-2)”); and

(G-3) polyamine having at least one C₄₀₋₄₀₀ alkyl or alkenyl group in its molecule, or derivatives thereof (hereinafter may be referred to as “component (G-3)”).

The component (G-1) may be especially preferably employed as the component (G).

Examples of succinimide having at least one C₄₀₋₄₀₀ alkyl or alkenyl group in its molecule among the component (G-1) include compounds represented by the following general formula (29) or (30):

wherein in the formula (29), R⁵⁶ is a C₄₀-C₄₀₀ alkyl or alkenyl group; q is an integer of 1 to 5, which is preferably 2 to 4. The carbon number of R⁵⁶ is preferably no less than 60, and preferably no more than 350.

In the formula (30), R⁵⁷ and R⁵⁸ are each independently a C₄₀-C₄₀₀ alkyl or alkenyl group, and may be combination of different groups. R⁵⁷ and R⁵⁸ are especially preferably polybutenyl groups. In addition, r is an integer of 0 to 4, which is preferably 1 to 3. The carbon numbers of R⁵⁷ and R⁵⁸ are preferably no less than 60, and preferably no more than 350.

R⁵⁶ to R⁵⁸ in the formulae (29) and (30) having carbon numbers of this lower limit or over offers good solubility in the lubricating base oil. R⁵⁶ to R⁵⁸ having carbon numbers of this upper limit or below offers improved low-temperature fluidity of the lubricating oil composition.

The alkyl or alkenyl group (R⁵⁶ to R⁵⁸) in the formulae (29) and (30) may be linear or branched. Preferred examples thereof include branched alkyl groups and branched alkenyl groups derived from oligomers of olefins such as propylene, 1-butene, and isobutene, or from co-oligomers of ethylene and propylene. Among them, a branched alkyl or alkenyl group derived from an oligomer of isobutene which is conventionally referred to as polyisobutylene, or a polybutenyl group is most preferable.

A preferred number average molecular weight of the alkyl or alkenyl group (R⁵⁶ to R⁵⁸) in the formulae (29) and (30) is 800 to 3500.

The succinimide having at least one alkyl or alkenyl group in its molecule encompasses so-called mono-type succinimide represented by the formula (29) wherein addition of succinic anhydride has occurred at only one terminal position of a polyamine chain, and so-called bis-type succinimide represented by the formula (30) wherein succinic anhydride terminates both ends of a polyamine chain. The lubricating oil composition may comprise either mono-type or bis-type succinimide, or may comprise both of them as a mixture.

A method for producing the succinimide having at least one alkyl or alkenyl group in its molecule is not specifically limited. For example, the succinimide can be obtained by: making alkyl succinic acid or alkenyl succinic acid react with a polyamine, the alkyl or alkenyl succinic acid being obtained by making a compound having a C₄₀₋₄₀₀ alkyl or alkenyl group react with maleic anhydride at 100 to 200° C. Here, examples of the polyamine include diethylenetriamine, triethylenetetramine, tetraethylenepentamine, and pentaethylenehexamine.

Examples of benzylamine having at least one C₄₀₋₄₀₀ alkyl or alkenyl group in its molecule among the component (G-2) include compounds represented by the following general formula (31):

wherein in the formula (31), R⁵⁹ is a C₄₀₋₄₀₀ alkyl or alkenyl group; and s is an integer of 1 to 5, which is preferably 2 to 4. The carbon number of R⁵⁹ is preferably no less than 60, and preferably no more than 350.

A method for producing the component (G-2) is not specifically limited. Examples of such a method include: making a polyolefin such as a propylene oligomer, polybutene, and an ethylene-α-olefin copolymer, react with a phenol, to give an alkylphenol; and then making formaldehyde, and a polyamine such as diethylenetriamine, triethylenetetramine, tetraethylenepentamine, and pentaethylenehexamine, react with the alkylphenol by Mannich reaction.

Examples of a polyamine having at least one C₄₀₋₄₀₀ alkyl or alkenyl group in its molecule among the component (G-3) include compounds represented by the following formula (32):

R⁶⁰-NH—(CH₂CH₂NH)_(t)—H   (32)

wherein in the formula (32), R⁶⁰ is a C₄₀₋₄₀₀ alkyl or alkenyl group; and t is an integer of 1 to 5, which is preferably 2 to 4. The carbon number of R⁶⁰ is preferably no less than 60, and preferably no more than 350.

A method for producing the component (G-3) is not specifically limited. Examples of such a method include: chlorinating a polyolefin such as a propylene oligomer, polybutene, and an ethylene-α-olefin copolymer; and then making ammonia, or a polyamine such as ethylenediamine, diethylenetriamine, triethylenetetramine, tetraethylenepentamine, and pentaethylenehexamine react with the chlorinated polyolefin.

Examples of derivatives among the components (G-1) to (G-3) include (i) oxygen-containing organic compound-modified products wherein a part or all of the residual amino groups and/or imino groups is/are neutralized or amidated by making a C₁₋₃₀ monocarboxylic acid such as fatty acids, a C₂₋₃₀ polycarboxylic acid (such as ethanedioic acid, phthalic acid, trimellitic acid, and pyromellitic acid), an anhydride or ester thereof, a C₂₋₆ alkylene oxide, or a hydroxy(poly)oxyalkylene carbonate react with the succinimide, benzylamine or polyamine having at least one alkyl or alkenyl group in its molecule (hereinafter referred to as “the above described nitrogen-containing compound”); (ii) boron-modified products wherein a part or all of the residual amino groups and/or imino groups is/are neutralized or amidated by making boric acid react with the above described nitrogen-containing compound; (iii) phosphoric acid-modified products wherein a part or all of the residual amino groups and/or imino groups is/are neutralized or amidated by making the above described nitrogen-containing compound react with phosphoric acid; (iv) sulfur-modified products obtained by making a sulfur compound react with the above described nitrogen-containing compound; and (v) modified products obtained by at least two modifications selected from the oxygen-containing organic compound modification, boron modification, phosphoric acid modification, and sulfur modification, on the above described nitrogen-containing compound in combination. Among these derivatives (i) to (v), a boron-modified product of the component (G-1) is preferably used in view of making it possible to further improve thermal stability of the lubricating oil composition.

The molecular weight of the component (G) is not specifically restricted, and the weight-average molecular weight thereof is preferably 1000 to 20000.

When the lubricating oil composition comprises the component (G), the content thereof is, in terms of nitrogen on the basis of the total mass of the lubricating oil composition, preferably 30 to 300 mass ppm, more preferably no less than 50 mass ppm, and more preferably no more than 230 mass ppm. The content of the component (G) of this lower limit or over makes it possible to improve thermal stability of the lubricating oil composition. The content of the component (G) of this upper limit or below makes it possible to further improve fuel efficiency.

When a boron-modified product is used as the component (G), the boron content derived from the component (G) in the lubricating oil composition is, on the basis of the total mass of the lubricating oil composition, preferably 50 to 500 mass ppm, more preferably no less than 100 mass ppm, and more preferably no more than 300 mass ppm. The boron content derived from the component (G) of this upper limit or below makes it possible to further improve fuel efficiency.

<Other Additives>

The lubricating oil composition of the present invention may further comprise at least one additive selected from an antioxidant, a corrosion inhibitor other than the component (E), an anti-rust agent, a metal deactivator other than the component (E), a defoaming agent, a demulsifier, and a coloring agent.

Examples of the antioxidant include a phenolic or amine ashless antioxidant, and a copper or molybdenum metallic antioxidant. Specific examples of a phenolic ashless antioxidant include 4,4′-methylenebis(2,6-di-tert-butylphenol), and 4,4′-bis(2,6-di-tert-butylphenol); and examples of an amine ashless antioxidant include phenyl-α-naphthylamine, alkylphenyl-α-naphthylamine, and dialkyldiphenylamine. When the lubricating oil composition comprises the antioxidant, the content thereof is usually 0.01 to 5 mass % on the basis of the total mass of the lubricating oil composition.

Any known corrosion inhibitor such as a benzotriazole, tolyltriazole, and imidazole compound may be employed as the corrosion inhibitor other than the component (E). When the lubricating oil composition comprises the corrosion inhibitor other than the component (E), the content thereof is usually 0.005 to 5 mass % on the basis of the total mass of the lubricating oil composition.

Any known anti-rust agent such as petroleum sulfonate, alkylbenzenesulfonate, dinonylnaphthalenesulfonate, alkenylsuccinate esters, and polyol esters may be employed as the anti-rust agent. When the lubricating oil composition comprises the anti-rust agent, the content thereof is usually 0.005 to 5 mass % on the basis of the total mass of the lubricating oil composition.

Any known metal deactivator such as imidazoline, pyrimidine derivatives, mercaptobenzothiazole, benzotriazole and its derivatives, 2-(alkyldithio)benzimidazole, and β-(o-carboxybenzylthio)propionitrile may be employed as the metal deactivator other than the component (E). When the lubricating oil composition comprises the metal deactivator other than the component (E), the content thereof is usually 0.005 to 5 mass % on the basis of the total mass of the lubricating oil composition.

Any known defoaming agent such as silicones, fluorosilicones, and fluoroalkyl ethers may be employed as the defoaming agent. When the lubricating oil composition comprises the defoaming agent, the content thereof is usually 0.0005 to 0.01 mass % on the basis of the total mass of the lubricating oil composition.

Any known demulsifier such as polyalkylene glycol nonionic surfactants may be employed as the demulsifier. When the lubricating oil composition comprises the demulsifier, the content thereof is usually 0.005 to 5 mass % on the basis of the total mass of the lubricating oil composition.

As the coloring agent, any known coloring agent such as azo compounds may be employed.

<Lubricating Oil Composition>

The kinematic viscosity of the lubricating oil composition at 100° C. is preferably 4.5 to 7.5 mm²/s. The kinematic viscosity of the lubricating oil composition at 100° C. of no more than 7.5 mm²/s makes it easy to improve low temperature viscosity characteristics and fuel efficiency of the lubricating oil composition. The kinematic viscosity of the lubricating oil composition at 100° C. of no less than 4.5 mm²/s offers improved film retention under high temperature conditions, which makes it easy to improve anti-seizure, anti-wear, and fatigue life properties.

The kinematic viscosity of the lubricating oil composition at 40° C. is preferably 18 to 24 mm²/s. The kinematic viscosity of the lubricating oil composition at 40° C. of no more than 24 mm²/s makes it easy to improve fuel efficiency. The kinematic viscosity of the lubricating oil composition at 40° C. of no less than 18 mm²/s makes it easy to achieve enough oil film formation at a lubricating point and thus to improve anti-wear properties.

The viscosity index of the lubricating oil composition is preferably no less than 160. The upper limit of the viscosity index of the lubricating oil composition is not specifically restricted, and is usually no more than 300. The lubricating oil composition having a viscosity index of 160 or more makes it easy to improve fuel efficiency.

The Brookfield viscosity of the lubricating oil composition at −40° C. (hereinafter may be referred to as “BF viscosity”) is preferably less than 7500 mPa·s. When the BF viscosity of the lubricating oil composition is less than 7500 mPa·s at −40° C., low-temperature startability can be easily improved.

(Use)

The lubricating oil composition of the present invention can be preferably used as a lubricating oil of an apparatus comprising a wet clutch, and can be preferably used as a gear oil of a gear comprising a hypoid gear. The lubricating oil composition of the present invention can be particularly preferably used as a common lubricating oil of a wet clutch and a hypoid gear. Examples of such a use that the lubricating oil is used in common for lubricating a wet clutch and lubricating a hypoid gear include a common lubricating oil of an automatic transmission and a differential gear of an automobile.

EXAMPLES

Hereinafter the present invention will be more specifically described based on Examples and Comparative examples. It is noted that the present invention is not limited to these examples.

Examples 1 to 27 and Comparative Examples 1 to 7

The lubricating oil composition of the present invention (Examples 1 to 27) and lubricating oil compositions for comparison (Comparative examples 1 to 7) were prepared as shown in Tables 1 to 5. In Tables, the content of the base oil is on the basis of the total mass of the base oils, and the content of each additive is on the basis of the total mass of the composition. Details on the components are as follows:

(Lubricating Base Oil)

O1-1: Group II base oil of API base stock categories (Yubase™ 2 manufactured by SK Lubricants Co., Ltd.), kinematic viscosity (40° C.): 8.73 mm²/s, kinematic viscosity (100° C.): 2.435 mm²/s, viscosity index: 97, pour point: −40° C.

O1-2: Group IV base oil of API base stock categories (Durasyn™ 162 manufactured by INEOS Oligomers USA LLC), kinematic viscosity (40° C.): 5.361 mm²/s, kinematic viscosity (100° C.): 1.776 mm²/s, viscosity index: 89, sulfur content: less than 10 mass ppm

O1-3: Group II base oil of API base stock categories (ULTRA™ S-2 manufactured by S-OIL CORPORATION), kinematic viscosity (40° C.): 7.6 mm²/s, kinematic viscosity (100° C.): 2.259 mm²/s, viscosity index: 106, pour point: −35° C.

O2-1: Group III base oil of API base stock categories (Yubase™ 4 manufactured by SK Lubricants Co., Ltd.), kinematic viscosity (40° C.): 19.42 mm²/s, kinematic viscosity (100° C.): 4.234 mm²/s, viscosity index: 125, pour point: −17.5° C.

O2-2: Group IV base oil of API base stock categories (Durasyn™ 164 manufactured by INEOS Oligomers USA LLC), kinematic viscosity (40° C.): 17.60 mm²/s, kinematic viscosity (100° C.): 3.90 mm²/s, viscosity index: 122

O3-1: Group V base oil of API base stock categories (UNISTER™ MB881 manufactured by NOF CORPORATION), 2-ethylhexyl oleate, kinematic viscosity (40° C.): 8.4 mm²/s, kinematic viscosity (100° C.): 2.7 mm²/s, viscosity index: 179

O3-2: Group V base oil of API base stock categories (Synative™ ES2958 manufactured by BASF), diester, kinematic viscosity (40° C.): 10.3 mm²/s, kinematic viscosity (100° C.): 2.9 mm²/s, viscosity index: 138

((A) Dispersant Poly(meth)acrylate Compound)

A-1: dispersant polymethacrylate, weight average molecular weight: 40,000

A-2: dispersant polymethacrylate, weight average molecular weight: 150,000

((B) Non-dispersant Poly(meth)acrylate Compound)

B-1: non-dispersant polymethacrylate, weight average molecular weight: 20,000

B-2: non-dispersant polymethacrylate, weight average molecular weight: 30,000

B-3: non-dispersant polymethacrylate, weight average molecular weight: 50,000

B-4: non-dispersant polymethacrylate, weight average molecular weight: 80,000

B-5: non-dispersant polymethacrylate, weight average molecular weight: 120,000

B-6: non-dispersant polymethacrylate, weight average molecular weight: 400,000

((C) Phosphorus-containing Anti-wear Agent)

C-1: tricresyl phosphite, P: 8.4 mass %

C-2: dibutyl phosphite, P:15.5 mass %

C-3: phosphoric acid, P: 30.0 mass %

C-4: diphenyl hydrogen phosphite, P: 13.2 mass %

((D) Overbased Calcium Detergent)

D-1: overbased Ca salicylate, base number: 220 mgKOH/g, Ca: 8.1 mass %, average carbon number of alkyl groups: 22, metal ratio: 6.42

D-2: overbased Ca salicylate, base number: 320 mgKOH/g, Ca: 11.40 mass %, average carbon number of alkyl groups: 22, metal ratio: 13.31

((E) Sulfur Additive)

E-1: thiadiazole compound having hydrocarbyldithio groups represented by any of the general formulae (12) to (14), S:36 mass %

E-2: an alkyloxy- or alkenyloxy-substituted cyclic sulfone compound represented by the general formula (20), f=1, number average carbon number of R⁴⁵: 10, R⁴⁵O— substituent was in 3-position, S: 11.6 mass %

E-3: polysulfide, 5: 30.5 mass %

((F) Ashless Friction Modifier)

F-1: N,N-dilauryl-2-hydroxyacetamide

F-2: succinimide compound of the general formula (24) wherein R⁴⁹ to R⁵⁰=C₁₈ alkenyl groups, and h=1

F-3: a mixture of acylated polyamines of the general formula (26) wherein R⁵² to R⁵³=C₁₇ alkyl groups, i: integer of 1 to 2, j: integer of 0 to 2, k: integer of 0 to 1, j+k: integer of 1 to 2, and i+j+2k=3

(Other Additives)

Ashless dispersant: boron-containing succinimide having a polybutenyl group of number average molecular weight of 1300, bis-type, N: 0.322 mass %, B: 0.5 mass %

Antioxidant: amine antioxidant

Defoaming agent: dimethyl silicone defoaming agent, kinematic viscosity (25° C.): 60,000 mm²/s

TABLE 1 Examples 1 2 3 4 5 6 7 Base oil O1-1 mass % 75 75 75 75 75 75 75 O1-2 mass % O1-3 mass % O2-1 mass % 25 25 25 25 25 25 25 O2-2 mass % O3-1 mass % O3-2 mass % Viscosity characteristics of base oil Kinematic viscosity (40° C.) mm²/s 10.5 10.5 10.5 10.5 10.5 10.5 10.5 Kinematic viscosity (100° C.) mm²/s 2.76 2.76 2.76 2.76 2.76 2.76 2.76 Viscosity index 105 105 105 105 105 105 105 (A) Dispersant poly(meth)acrylate A-1 mass % 3.0 3.0 3.0 3.0 5.0 5.0 1.0 A-2 mass % (B) Non-dispersant poly(meth)acrylate B-1 mass % 8.7 5.6 B-2 mass % 7.6 5.1 10.3 B-3 mass % 5.0 B-4 mass % 3.6 B-5 mass % B-6 mass % (C) Phosphorus-containing anti-wear agent C-1 mass % C-2 mass % C-3 mass % 0.07 0.07 0.07 0.07 0.07 0.07 0.07 C-4 mass % 0.30 0.30 0.30 0.30 0.30 0.30 0.30 (D) Overbased Ca detergent D-1 mass % 0.12 0.12 0.12 0.12 0.12 0.12 0.12 D-2 mass % (E) Sulfur additive E-1 mass % 0.15 0.15 0.15 0.15 0.15 0.15 0.15 E-2 mass % 0.50 0.50 0.50 0.50 0.50 0.50 0.50 E-3 mass % (F) Ashless friction modifier F-1 mass % 3.0 3.0 3.0 3.0 3.0 3.0 3.0 F-2 mass % F-3 mass % Other additives Ashless dispersant mass % 4.5 4.5 4.5 4.5 4.5 4.5 4.5 Antioxidant mass % 0.5 0.5 0.5 0.5 0.5 0.5 0.5 Defoaming agent mass % 0.003 0.003 0.003 0.003 0.003 0.003 0.003 Viscosity characteristics of composition Kinematic viscosity (40° C.) mm²/s 22.0 21.5 21.3 22.0 21.6 21.5 22.0 Kinematic viscosity (100° C.) mm²/s 5.58 5.52 5.52 5.54 5.51 5.52 5.56 Viscosity index 212 215 218 209 212 215 207 P content from component (C) mass ppm 606 606 606 606 606 606 606 Ca content from component (D) mass ppm 97 97 97 97 97 97 97 S content from component (E) mass ppm 1120 1120 1120 1120 1120 1120 1120 MA/MB 0.344 0.397 0.596 0.833 0.901 0.990 0.098 MwB/MwA 0.500 0.750 1.250 2.000 0.500 0.750 0.750 MTM test: friction coefficient 0.026 0.025 0.034 0.038 0.030 0.026 0.029 EHL test: oil film thickness nm 14.3 14.5 15.2 12.5 12.6 12.5 13.5 BF viscosity (−40° C.) mPa · s 4700 4800 4700 4600 5600 5500 4200 ISOT test: increase of acid number mgKOH/g 0.59 0.53 0.53 0.53 0.49 0.54 0.51 Unisteel test: fatigue life F10 ×10⁶ rot. 2.6 3.0 3.2 2.6 2.5 2.6 2.5 High-speed four-ball test: wear mark mm 0.54 0.52 0.64 0.50 0.61 0.59 0.59 FALEX test: seizure load lbf 1850 1900 1920 1660 1820 1710 1750 Shudder prevention dμ/dV (after 48 h endurance) s/m 3.56 3.89 3.76 3.89 3.89 3.98 3.85 Shudder prevention lifetime hr 408 408 408 360 408 408 408

TABLE 2 Examples 8 9 10 11 12 13 14 Base oil O1-1 mass % 75 75 75 75 35 O1-2 mass % 40 O1-3 mass % 60 O2-1 mass % 25 25 25 25 40 O2-2 mass % 65 60 O3-1 mass % O3-2 mass % Viscosity characteristics of base oil Kinematic viscosity (40° C.) mm²/s 10.5 10.5 10.5 10.5 20.5 10.3 10.7 Kinematic viscosity (100° C.) mm²/s 2.76 2.76 2.76 2.76 4.35 2.74 2.84 Viscosity index 105 105 105 105 122 107 112 (A) Dispersant poly(meth)acrylate A-1 mass % 8.0 3.0 3.0 3.0 A-2 mass % 2.0 3.0 2.0 (B) Non-dispersant poly(meth)acrylate B-1 mass % 6.4 3.2 B-2 mass % 11.1 5.5 4.5 7.6 7.3 B-3 mass % B-4 mass % B-5 mass % B-6 mass % (C) Phosphorus-containing anti-wear agent C-1 mass % C-2 mass % C-3 mass % 0.07 0.07 0.07 0.07 0.07 0.07 0.07 C-4 mass % 0.30 0.30 0.30 0.30 0.30 0.30 0.30 (D) Overbased Ca detergent D-1 mass % 0.12 0.12 0.12 0.12 0.12 0.12 0.12 D-2 mass % (E) Sulfur additive E-1 mass % 0.15 0.15 0.15 0.15 0.15 0.15 0.15 E-2 mass % 0.50 0.50 0.50 0.50 0.50 0.50 0.50 E-3 mass % (F) Ashless friction modifier F-1 mass % 3.0 3.0 3.0 3.0 3.0 3.0 3.0 F-2 mass % F-3 mass % Other additives Ashless dispersant mass % 4.5 4.5 4.5 4.5 4.5 4.5 4.5 Antioxidant mass % 0.5 0.5 0.5 0.5 0.5 0.5 0.5 Defoaming agent mass % 0.003 0.003 0.003 0.003 0.003 0.003 0.003 Viscosity characteristics of composition Kinematic viscosity (40° C.) mm²/s 28.0 22.1 22.1 21.5 26.5 21.8 21.5 Kinematic viscosity (100° C.) mm²/s 7.48 5.56 5.51 5.50 6.50 5.57 5.55 Viscosity index 255 209 205 213 215 214 219 P content from component (C) mass ppm 606 606 606 606 606 606 606 Ca content from component (D) mass ppm 97 97 97 97 97 97 97 S content from component (E) mass ppm 1120 1120 1120 1120 1120 1120 1120 MA/MB 0.721 0.311 0.946 0.364 0.667 0.395 0.411 MwB/MwA 0.750 0.133 0.133 0.200 0.750 0.750 0.750 MTM test: friction coefficient 0.020 0.036 0.037 0.035 0.031 0.021 0.024 EHL test: oil film thickness nm 20.1 12.1 12.6 12.6 14.3 15.2 14.7 BF viscosity (−40° C.) mPa · s 7200 6500 6300 6400 6100 4100 5400 ISOT test: increase of acid number mgKOH/g 0.50 0.44 0.49 0.49 0.50 0.52 0.56 Unisteel test: fatigue life F10 ×10⁶ rot. 3.5 2.2 2.1 2.2 2.0 2.9 3.0 High-speed four-ball test: wear mark mm 0.51 0.63 0.65 0.62 0.60 0.55 0.52 FALEX test: seizure load lbf 1980 1730 1590 1750 1780 1900 1900 Shudder prevention dμ/dV (after 48 h endurance) s/m 4.86 3.57 3.68 3.96 3.01 3.90 3.89 Shudder prevention lifetime hr 456 408 384 408 408 408 40

TABLE 3 Examples 15 16 17 18 19 20 21 Base oil O1-1 mass % 70 70 75 75 75 75 75 O1-2 mass % O1-3 mass % O2-1 mass % 20 20 25 25 25 25 25 O2-2 mass % O3-1 mass % 10 O3-2 mass % 10 Viscosity characteristics of base oil Kinematic viscosity (40° C.) mm²/s 10.0 10.3 10.5 10.5 10.5 10.5 10.5 Kinematic viscosity (100° C.) mm²/s 2.72 2.74 2.76 2.76 2.76 2.76 2.76 Viscosity index 110 107 105 105 105 105 105 (A) Dispersant poly(meth)acrylate A-1 mass % 3.0 3.0 3.0 3.0 3.0 3.0 3.0 A-2 mass % (B) Non-dispersant poly(meth)acrylate B-1 mass % B-2 mass % 7.6 7.6 7.6 7.6 7.6 7.6 7.6 B-3 mass % B-4 mass % B-5 mass % B-6 mass % (C) Phosphorus-containing anti-wear agent C-1 mass % 0.250 C-2 mass % 0.135 C-3 mass % 0.07 0.07 0.24 0.07 0.07 C-4 mass % 0.30 0.30 0.30 0.30 0.30 0.30 0.30 (D) Overbased Ca detergent D-1 mass % 0.12 0.12 0.12 0.12 0.12 0.37 0.07 D-2 mass % (E) Sulfur additive E-1 mass % 0.15 0.15 0.15 0.15 0.15 0.15 0.15 E-2 mass % 0.50 0.50 0.50 0.50 0.50 0.50 0.50 E-3 mass % (F) Ashtess friction modifier F-1 mass % 3.0 3.0 3.0 3.0 3.0 3.0 3.0 F-2 mass % F-3 mass % Other additives Ashless dispersant mass % 4.5 4.5 4.5 4.5 4.5 4.5 4.5 Antioxidant mass % 0.5 0.5 0.5 0.5 0.5 0.5 0.5 Defoaming agent mass % 0.003 0.003 0.003 0.003 0.003 0.003 0.003 Viscosity characteristics of composition Kinematic viscosity (40° C.) mm²/s 21.5 21.6 21.6 21.5 21.5 21.5 21.6 Kinematic viscosity (100° C.) mm²/s 5.58 5.60 5.52 5.53 5.52 5.51 5.53 Viscosity index 220 220 213 214 216 214 214 P content from component (C) mass ppm 606 606 606 605 1106 606 606 Ca content from component (D) mass ppm 97 97 97 97 97 300 53 S content from component (E) mass ppm 1120 1120 1120 1120 1120 1120 1120 MA/MB 0.395 0.395 0.397 0.397 0.397 0.397 0.397 MwB/MwA 0.750 0.750 0.750 0.750 0.750 0.750 0.750 MTM test: friction coefficient 0.023 0.024 0.031 0.027 0.026 0.026 0.026 EHL test: oil film thickness nm 14.7 14.6 14.5 14.5 14.5 14.5 14.5 BF viscosity (−40° C.) mPa · s 4500 4500 4800 4800 4800 4800 4800 ISOT test: increase of acid number mgKOH/g 0.64 0.75 0.39 0.72 0.60 0.67 0.48 Unisteel test: fatigue life F10 ×10⁶ rot. 3.1 3.1 2.2 2.8 3.0 2.7 2.6 High-speed four-ball test: wear mark mm 0.51 0.52 0.68 0.53 0.46 0.62 0.55 FALEX test: seizure load lbf 1900 1900 1530 1850 2060 1700 1750 Shudder prevention dμ/dV (after 48 h endurance s/m 3.56 3.78 3.76 3.85 3.98 3.97 2.26 Shudder prevention lifetime hr 408 408 360 456 408 432 240

TABLE 4 Examples 22 23 24 25 26 27 Base oil O1-1 mass % 75 75 75 75 75 75 O1-2 mass % O1-3 mass % O2-1 mass % 25 25 25 25 25 25 O2-2 mass % O3-1 mass % O3-2 mass % Viscosity characteristics of base oil Kinematic viscosity (40° C.) mm²/s 10.5 10.5 10.5 10.5 10.5 10.5 Kinematic viscosity (100° C.) mm²/s 2.76 2.76 2.76 2.76 2.76 2.76 Viscosity index 105 105 105 105 105 105 (A) Dispersant poly(meth)acrylate A-1 mass % 3.0 3.0 3.0 3.0 3.0 3.0 A-2 mass % (B) Non-dispersant poly(meth)acrylate B-1 mass % B-2 mass % 7.6 7.6 7.6 7.6 7.6 7.6 B-3 mass % B-4 mass % B-5 mass % B-6 mass % (C) Phosphorus-containing anti-wear agent C-1 mass % C-2 mass % C-3 mass % 0.07 0.07 0.07 0.07 0.07 0.07 C-4 mass % 0.30 0.30 0.30 0.30 0.30 0.30 (D) Overbased Ca detergent D-1 mass % 0.12 0.12 0.12 0.12 0.12 D-2 mass % 0.09 (E) Sulfur additive E-1 mass % 0.15 0.15 0.15 0.05 0.15 0.15 E-2 mass % 0.50 0.50 0.23 0.81 0.50 0.50 E-3 mass % 0.06 (F) Ashless friction modifier F-1 mass % 3.0 3.0 3.0 3.0 F-2 mass % 3.0 F-3 mass % 1.0 Other additives Ashless dispersant mass % 4.5 4.5 4.5 4.5 4.5 4.5 Antioxidant mass % 0.5 0.5 0.5 0.5 0.5 0.5 Defoaming agent mass % 0.003 0.003 0.003 0.003 0.003 0.003 Viscosity characteristics of composition Kinematic viscosity (40° C.) mm²/s 21.5 21.5 21.4 21.3 21.5 21.3 Kinematic viscosity (100° C.) mm²/s 5.50 5.52 5.51 5.50 5.52 5.49 Viscosity index 213 215 216 216 214 215 P content from component (C) mass ppm 606 606 606 606 606 606 Ca content from component (D) mass ppm 97 97 97 97 97 97 S content from component (E) mass ppm 1120 1297 807 1120 1120 1120 MA/MB 0.397 0.397 0.397 0.397 0.397 0.397 MwB/MwA 0.750 0.750 0.750 0.750 0.750 0.750 MTM test: friction coefficient 0.032 0.026 0.026 0.027 0.025 0.025 EHL test: oil film thickness nm 14.5 14.5 14.5 14.5 14.5 14.5 BF viscosity (−40° C.) mPa · s 4800 4800 4800 4800 4800 4800 ISOT test: increase of acid number mgKOH/g 0.50 0.98 0.30 0.66 0.53 0.53 Unisteel test: fatigue life F10 ×10⁶ rot. 2.2 3.0 2.6 2.1 3.0 3.0 High-speed four-ball test: wear mark mm 0.58 0.51 0.65 0.63 0.52 0.52 FALEX test: seizure load lbf 1600 1950 1620 1590 1900 1900 Shudder prevention dμ/dV (after 48 h endurance) s/m 4.05 2.23 3.98 3.05 3.54 3.12 Shudder prevention lifetime hr 432 216 408 384 384 264

TABLE 5 Comparative examples 1 2 3 4 5 6 7 Base oil O1-1 mass % 75 75 75 75 75 75 75 O1-2 mass % O1-3 mass % O2-1 mass % 25 25 25 25 25 25 25 O2-2 mass % O3-1 mass % O3-2 mass % Viscosity characteristics of base oil Kinematic viscosity (40° C.) mm²/s 10.5 10.5 10.5 10.5 10.5 10.5 10.5 Kinematic viscosity (100° C.) mm²/s 2.76 2.76 2.76 2.76 2.76 2.76 2.76 Viscosity index 105 105 105 105 105 105 105 (A) Dispersant poly(meth)acrylate A-1 mass % 8.3 3.0 7.0 3.0 A-2 mass % 3.5 (B) Non-dispersant poly(meth)acrylate B-1 mass % 10.0 B-2 mass % 2.5 11.3 2.4 B-3 mass % B-4 mass % B-5 mass % 4.4 B-6 mass % 1.0 (C) Phosphorus-containing anti-wear agent C-1 mass % C-2 mass % C-3 mass % 0.07 0.07 0.07 0.07 0.07 0.07 0.07 C-4 mass % 0.30 0.30 0.30 0.30 0.30 0.30 0.30 (D) Overbased Ca detergent D-1 mass % 0.12 0.12 0.12 0.12 0.12 0.12 0.12 D-2 mass % (E) Sulfur additive E-1 mass % 0.15 0.15 0.15 0.15 0.15 0.15 0.15 E-2 mass % 0.50 0.50 0.50 0.50 0.50 0.50 0.50 E-3 mass % (F) Ashless friction modifier F-1 mass % 3.0 3.0 3.0 3.0 3.0 3.0 3.0 F-2 mass % F-3 mass % Other additives Ashless dispersant mass % 4.5 4.5 4.5 4.5 4.5 4.5 4.5 Antioxidant mass % 0.5 0.5 0.5 0.5 0.5 0.5 0.5 Defoaming agent mass % 0.003 0.003 0.003 0.003 0.003 0.003 0.003 Viscosity characteristics of composition Kinematic viscosity (40° C.) mm²/s 22.8 22.6 24.0 23.5 22.6 21.7 22.0 Kinematic viscosity (100° C.) mm²/s 5.33 5.24 5.45 5.52 5.46 5.51 5.54 Viscosity index 180 175 174 186 193 211 209 P content from component (C) mass ppm 606 606 606 606 606 606 606 Ca content from component (D) mass ppm 97 97 97 97 97 97 97 S content from component (E) mass ppm 1120 1120 1120 1120 1120 1120 1120 MA/MB — — 3.000 2.800 0.682 0.000 0.000 MwB/MwA — — 10.000 0.750 3.000 — — MTM test: friction coefficient 0.042 0.045 0.046 0.042 0.048 0.070 0.066 EHL test: oil film thickness nm 9.4 8.1 8.5 11.3 8.8 10.5 11.1 BF viscosity (−40° C.) mPa · s 6400 8100 8700 5100 8200 4900 5400 ISOT test: increase of acid number mgKOH/g 0.49 0.58 0.67 0.48 0.67 0.50 0.47 Unisteel test: fatigue life F10 ×10⁶ rot. 1.8 1.6 1.7 1.9 1.7 1.7 1.6 High-speed four-ball test: wear mark mm 0.75 0.72 0.63 0.63 0.64 0.72 0.75 FALEX test: seizure load lbf 1430 1310 1530 1640 1680 1320 1520 Shudder prevention dμ/dV (after 48 h endurance) s/m 3.44 3.86 3.53 3.78 3.51 3.23 3.55 Shudder prevention lifetime hr 264 240 384 384 288 288 288

(MTM Test)

A ball-on-disk friction test was carried out on each of the lubricating oil compositions using a MTM traction machine (manufactured by PCS Instruments), to measure friction coefficient. The measurement conditions were as follows:

ball and disk: standard test pieces (AISI 52100 standard)

oil temperature: 120° C.

load: 50 N

speed: 1 m/s

slip rate: 50%

The results are shown in Tables 1 to 5. Friction coefficients measured in this test are those not under hydrodynamic lubrication conditions but in a transition from hydrodynamic lubrication to boundary lubrication (mixed lubrication wherein an oil film is thin). The friction coefficient measured in this test of no more than 0.04 means that friction was sufficiently reduced in the mixed lubrication as well.

(EHL Test)

For each lubricating oil composition, oil film thickness under an elastohydrodynamic lubrication condition was measured by optical interferometry using an EHL testing machine (EHD2 ultra thin film measurement system manufactured by PCS Instruments). The measurement conditions were as follows:

steel ball: Standard Ball (material: SUJ-2) manufactured by PCS Instruments, diameter: 19.05 mm

disc: a glass disc comprising a glass substrate, a Cr layer coated on the surface of the glass substrate, and a silica layer coated on the surface of the Cr layer

oil temperature: 120° C.

load: 20 N

average Hertz pressure: 0.5 GPa

speed: 0.1 m/s

slip rate: 10%

The results are shown in Tables 1 to 5. The oil film thickness measured in this test of no less than 10 nm means sufficient oil film thickness.

(Low-temperature Viscosity Characteristics)

For each lubricating oil composition, viscosity at −40° C. in oil temperature (BF viscosity) was measured using a Brookfield viscometer. The results are shown in Tables 1 to 5. The BF viscosity at −40° C. of less than 7500 mPa·s means good low-temperature viscosity characteristics.

(ISOT Oxidation Stability Test)

Oxidation stability of each of the lubricating oil compositions was evaluated by an ISOT test conforming to JIS K2514. The test was carried out at 150° C. in oil temperature for 120 hours, to measure the increase of the acid number (mgKOH/g) after the test. The results are shown in Tables 1 to 5. The increase of the acid number in this test of less than 1 mgKOH/g means good oxidation stability.

(Unisteel Test)

For each lubricating oil composition, rolling fatigue life of a thrust needle roller bearing was measured by a Unisteel test (IP305/79, The Institute of Petroleum) using a Unisteel rolling fatigue testing machine (triple-type high-temperature rolling fatigue testing machine (TRF-1000/3-01H) manufactured by Tokyo Koki Testing Machine Co. Ltd.). The rotation number of a bearing until a test piece of either a roller or the lace suffers fatigue damage was measured for a test bearing made by replacing a bearing ring in one side of a thrust needle bearing (FNTA-2542C manufactured by NSK Ltd.) with a flat test piece (lace) (material: SUJ2), under conditions of: 700 N in load; 2.0 GPa in surface pressure; 1410 rpm in rotation number; and 110° C. in oil temperature. It was determined that fatigue damage occurred when the vibration acceleration of a testing portion measured by a vibration accelerometer installed in the Unisteel rolling fatigue testing machine reached 1.5 m/s². The test was repeated ten times, and then the fatigue life was calculated as the 10% life (F10: rotation number for the cumulative probability of fatigue damage to be 10%) by a Weibull plot based on the time it had taken for fatigue damage to occur in the tests. The results are shown in Tables 1 to 5. The 10% life measured in these tests of no less than 2.0×10⁶ rotation numbers means good fatigue life.

(High-speed Four-ball Test)

For each lubricating oil composition, anti-wear properties of the lubricating oil composition were evaluated by a high-speed four-ball test conforming to JPI-5S-40-93. The size of a wear mark after driving at 1500 rpm in rotation number at 392 N in load and 100° C. in oil temperature for 30 minutes was measured. The results are shown in Tables 1 to 5. The size of a wear mark of no more than 0.70 mm in this test means good anti-wear properties.

(FALEX Seizure Test)

For each lubricating oil composition, anti-seizure properties were evaluated by a FALEX seizure test conforming to ASTM D3233 A. Under the condition of oil temperature at 110° C., a steel pin that was held by two stationary steel V-shaped blocks was rotated at 290 rpm, and the load at which seizure occurred was measured. The results are shown in Tables 1 to 5. The load at which seizure occurred of no less than 1500 lbf means good anti-seizure properties.

(Shudder Prevention Test)

Shudder prevention and shudder prevention lifetime of each of the lubricating oil compositions were evaluated by means of a low speed sliding testing machine specified in JASO M349: 2010. The testing method conformed to JASO M349: 2010. In the test, a μ-V curve was measured at an oil temperature of 80° C. Conforming to JASO M315: 2004, the shudder prevention lifetime was determined by: approximating the measured μ-V curve with a quintic function by least square method; differentiating the approximate function at the point of sliding speed (V) of 0.9 m/s; and determining the point of time when the gradient value became negative as the end of lifetime. The results are shown in Tables 1 to 5. A positive gradient value after 48-hour endurance in this test means good shudder prevention at the initial stage of use. The shudder prevention lifetime of no less than 200 hours in this test means good shudder prevention lifetime.

(Evaluation Results)

The lubricating oil compositions of Examples 1 to 27 showed good results in the MTM test (friction coefficient in the mixed lubrication), the EHL test (oil film thickness), low-temperature viscosity characteristics, oxidation stability, fatigue life, anti-wear properties, anti-seizure properties (load capacity), and shudder prevention for a wet clutch.

The lubricating oil composition of Comparative example 1, which did not comprise the component (B), showed inferior results in friction coefficient in the mixed lubrication, oil film thickness, fatigue life, anti-wear properties, and anti-seizure properties.

The lubricating oil composition of Comparative example 2, which did not comprise the component (B), showed inferior results in friction coefficient in the mixed lubrication, oil film thickness, low-temperature viscosity characteristics, fatigue life, anti-wear properties, and anti-seizure properties.

The lubricating oil composition of Comparative example 3, wherein the weight average molecular weight of the component (B) was too large, the ratio MwB/MwA of the weight average molecular weight of the component (A) and that of the component (B) was too large, and the ratio MA/MB of the amount of the component (A) and that of the component (B) was too large, showed inferior results in friction coefficient in the mixed lubrication, oil film thickness, low-temperature viscosity characteristics, and fatigue life.

The lubricating oil composition of Comparative example 4, wherein the ratio MA/MB of the amount of the component (A) and that of the component (B) was too large, showed inferior results in friction coefficient in the mixed lubrication, and fatigue life.

The lubricating oil composition of Comparative example 5, wherein the weight average molecular weight of the component (B) was too large, and the ratio MwB/MwA of the weight average molecular weight of the component (A) and that of the component (B) was too large, showed inferior results in friction coefficient in the mixed lubrication, oil film thickness, low-temperature viscosity characteristics, and fatigue life.

The lubricating oil composition of Comparative example 6, which did not comprise the component (A), and which had the too small ratio MA/MB of the amount of the component (A) and that of the component (B), showed inferior results in friction coefficient in the mixed lubrication, fatigue life, anti-wear properties, and anti-seizure properties.

The lubricating oil composition of Comparative example 7, which did not comprise the component (A), and which had the too small ratio MA/MB of the amount of the component (A) and that of the component (B), showed inferior results in friction coefficient in the mixed lubrication, fatigue life, and anti-wear properties. 

1. A lubricating oil composition comprising: a lubricating base oil; (A) a dispersant poly(meth)acrylate compound having a weight average molecular weight of 30,000 to 200,000 in an amount of 1 to 10 mass % on the basis of the total mass of the composition; and (B) a non-dispersant poly(meth)acrylate compound having a weight average molecular weight of 15,000 to 100,000 in an amount of no more than 15 mass % on the basis of the total mass of the composition, wherein a ratio MA/MB of the amount MA of the component (A) to the amount MB of the component (B) is 0.05 to 1; and a ratio MwB/MwA of the weight average molecular weight MwB of the component (B) to the weight average molecular weight MwA of the component (A) is 0.05 to
 2. 2. The lubricating oil composition according to claim 1, the lubricating base oil consisting of: at least one Group II mineral base oil of API base stock categories, at least one Group III mineral base oil of API base stock categories, at least one Group IV synthetic base oil of API base stock categories, or at least one Group V synthetic base oil of API base stock categories, or any combination thereof, wherein the lubricating base oil has a kinematic viscosity at 100° C. of 2.5 to 4.5 mm²/s.
 3. The lubricating oil composition according to claim 1, wherein the lubricating oil composition has a kinematic viscosity at 100° C. of 4.5 to 7.5 mm²/s.
 4. The lubricating oil composition according to claim 1, further comprising: (C) a phosphorus-containing anti-wear agent in an amount of 200 to 1110 mass ppm in terms of phosphorus on the basis of the total mass of the composition.
 5. The lubricating oil composition according to claim 1, further comprising: (D) an overbased calcium detergent in an amount of 50 to 300 mass ppm in terms of calcium on the basis of the total mass of the composition.
 6. The lubricating oil composition according to claim 5, the component (D) comprising an overbased calcium salicylate detergent in an amount of 50 to 300 mass ppm in terms of calcium on the basis of the total mass of the composition.
 7. The lubricating oil composition according to claim 1, further comprising: (E) a sulfur additive in an amount of 800 to 1300 mass ppm in terms of sulfur on the basis of the total mass of the composition.
 8. The lubricating oil composition according to claim 1, further comprising: (F) a compound comprising a C₁₀₋₂₄ chain aliphatic hydrocarbyl group and at least one functional group in an amount of 0.1 to 10.0 mass % on the basis of the total mass of the composition, the at least one functional group being selected from an amide bond, an imide bond, and an amino group.
 9. The lubricating oil composition according to claim 1, which is used in common for lubricating a wet clutch and lubricating a hypoid gear.
 10. A method for lubricating a wet clutch and a hypoid gear, the method comprising: supplying the lubricating oil composition as in claim 1 to the wet clutch; and supplying the lubricating oil composition to the hypoid gear, wherein the lubricating oil composition is used in common for lubricating the wet clutch and lubricating the hypoid gear. 